Angiogenesis, the production of new blood vessels from existing vasculature, is required for normal development, wound repair, and vascular homeostasis. Neuropilin is an essential mammalian cell surface receptor for one of the most potent pro-angiogenic molecules, vascular endothelial growth factor (VEGF), and functions in synchrony with VEGFR receptor tyrosine kinases. Neuropilin is of particular medical importance because it is critical to the pathology of VEGF dependent hyper-vascularization in tumor angiogenesis. We propose a series of parallel and complementary structural and functional studies to understand the physical interactions and receptor coupling at the heart of the VEGF signaling complex. Unlike many other kinase systems, VEGF binding to VEGFR in the absence of neuropilin is insufficient to induce angiogenesis. We seek to first understand the molecular mechanism(s) underlying neuropilin's unique role in VEGF dependant angiogenesis. In Aim 1, we will determine the basis for competitive binding of ligands to neuropilin, and the role that this competition plays in regulating angiogenesis. In Aim 2, we will explore the structural and conformational changes that neuropilin undergoes in its transition from inactive monomer to activated dimer. More globally, we seek to define the physical basis for the coordinated action of VEGF, neuropilin, and VEGFR. Aim 3 is focused on utilizing multiple structural and biochemical tools coupled with endothelial cell signaling assays to determine the physical interactions and conformational changes necessary to couple extracellular ligand binding to receptor activation. Taken together, these studies will provide a molecular understanding of the physical mechanisms underlying the critical first step connecting VEGF ligand binding to receptor activation.